Before investigating and treating problems with penis erection, it is necessary to find out whether a patient is in fact still having erections, and if so, what is the quality or the degree of erection when such occurs. For example, when a patient suffers from difficulties or problems in regard to erection, which are due to psychological reasons, it may happen that the patient experiences erections while asleep, although he himself knows nothing of such erections. In other cases however, before arriving at a decision as to whether for example it is appropriate or necessary to employ a penis prosthesis, it is necessary to carry out investigations into the quality of the erection achieved. That includes for example and in particular determining the increase in diameter or peripheral measurement of the penis, and also establishing the force with which such increase in penis size occurs, in order thereby to be able to arrive at conclusions concerning the degree of stiffness which is attained in the erected condition.